1. Field of the Invention
The present invention relates to multi-color electrophotographic machines, and, more particularly, to setting laser power and developer bias in multi-color electrophotographic machines.
2. Description of the Related Art
Toner patch sensors are used in color printers and copiers to monitor and control the amount of toner laid down by the electrophotographic process. Toner patch sensors reflect light off of a toner patch to determine how much toner was laid down during the electrophotographic process. The sensor's voltage signal from reading a toner patch is compared to the sensor signal from reading a bare surface to produce either a voltage difference or a ratio between the two signals. In either case, when the reflectivity of the bare surface changes due to wear or toner filming, the accuracy of the toner patch sensor's estimates of toner mass per unit area or fused image density is compromised.
Toner patch sensors are used in printers and copiers to monitor the toner density of unfused images and provide a means of controlling the print darkness. In color printers and copiers, the toner patch sensors are used to maintain the color balance and in some cases to modify the gamma correction or halftone linearization as the electrophotographic process changes with the environment and aging effects. Conventional reflection based toner sensors use a single light source to illuminate a test patch of toner and one or more photosensitive devices to detect the reflected light.
The cyan, magenta, yellow and black color planes can be accumulated on an intermediate belt. A single reflective sensor can be used to sense the toner density of special test patches formed and transferred onto the intermediate belt. The reflection signal of the test patches is a function of both the toner density in mg/cm2 and the reflectivity of the intermediate belt on which it rests. To properly interpret the reflection signals from the test patches, one must take into account the reflectivity of the intermediate belt. Unfortunately the reflectivity of the intermediate belt increases by 70–80% over life due to surface abrasion, toner filming, and the accumulation of toner fines and extra-particulates (fumed silica and titania). It is known to use a movable sensor in conjunction with a reference reflectivity surface that can be used to determine the reflectivity of the intermediate surface. However, this solution adds cost and complexity to the toner patch sensor.
What is needed in the art is an alternate method of estimating the reflectivity of the intermediate belt that does not increase the cost and complexity of the toner patch sensor hardware.